Calcium-regulated release of neurotransmitter is a fundamentally important process throughout the central nervous system. How this process is regulated presynaptically is not yet fully understood, but recent advances in the detection of synaptic vesicle fusion and neurotransmitter release, combined with the identification of putative proteins that either comprise or interact with the secretion machinery have now made such mechanistic investigations feasible. The long-range goal of this research project is to understand how the transfer of visual information between the outer and inner retina is regulated at the presynaptic level in retinal bipolar cells. The approach is to use a combination of biophysical techniques, including measurements of membrane capacitance and flash-photolysis of caged-calcium, in conjunction with probes designed to interact with SNARE proteins, to identify distinct stages of the secretion pathway in single synaptic terminals of retinal bipolar cells. The results of these experiments will guide the formation of a testable model of the late steps in neurotransmitter exocytosis at a ribbon synapse. Then, modulation of this secretory pathway by physiologically relevant neurotransmitters, neuropeptides, and second messenger systems will be examined to determine the extent to which they modulate synaptic release. Calcium, in addition to its role in initiating release, may effect other parts of the secretory pathway. Therefore, the actions of calcium and calcium- activated conductances on synaptic release and synaptic vesicle cycling will also be examined. Armed with this information, predictions will be made about the effects of local circuit interactions and previous stimulus history on the throughput of visual information from photoreceptors to the third-order neurons of the retina.